For many years various clays have been used in combination with both petroleum- and coal tar-derived bitumens to form aqueous emulsions. These so-called "colloidal clays" have many of the chemical and physical properties necessary to suspend the bitumen in the aqueous phase, providing the emulsion the consistency and stability desired.
Colloidal clay-stabilized emulsions do, however, have a tendency to phase-separate: the bitumen and clay particles (the discontinuous phase) settle over time, becoming a dense mass. Reblending into a homogeneous colloidal emulsion is often difficult and time-consuming. Phase-separation is especially troublesome and costly when extensive reblending is required just prior to application. The consequences of applying a phase-separated emulsion are even more undesirable and include: inadequate coverage rates, disabled spray equipment, non-uniform curing rates, varying cured-coating performance properties, and discolored and unsightly coating appearances, to mention but a few.
Early concern over phase separation and the problems associated with it fostered the use of water-soluble gums, including cellulose ethers, which retard phase separation by absorbing water from the aqueous phase and increasing emulsion viscosity. Essentially, the water is thickened to the extent that the modified fluid properties physically slow the settling of bitumen and clay particles.
Cellulose ethers, for example, are typically pre-mixed in a portion of the aqueous phase, which is later incorporated into the emulsion before application. As a whole, this approach is relatively inexpensive and alleviates many of the aforementioned concerns.
However, the prior art has associated with it a number of significant problems and deficiencies. Most are related to inadequate emulsion performance and phase-separation, and result from the emulsion thickeners currently used.
A number of significant problems stem from the fact that thickeners of the prior art are typically dry powders, which undergo immediate hydration upon contact with water. These gums tend to agglomerate and build localized viscosities which inhibit homogeneous dispersion. Because each gum particle must be hydrated, the final emulsion viscosity may be obtained only with days of continuous stirring. If a homogeneous blend is not obtained unhydrated gum may be applied with the emulsion. Because gums retain their hygroscopic effects once in the emulsion, contact with moisture after application invariably results in an uptake of water, re-emulsification, and a total breakdown of the cured film.
A related problem is overdosage. Because final emulsion viscosity lags so far behind the addition of thickener a concern is a tendency to add too much. A slight excess may produce an emulsion too thick and entirely unacceptable for a given application.
To overcome these and other problems, the particles of many gum-type thickeners of the prior art are chemically encapsulated during manufacture and pre-dispersed in a relatively small portion of the aqueous phase during the initial part of the preparation. The resulting slurry is added to a dispersion of clay and bitumen. De-encapsulation is accomplished by treatment with base, at which point viscosity develops rapidly.
This attempt to solve one problem creates several others. If a gum concentration is too high attempts to adjust the pH may lead to excessively high viscosities, so much so that the emulsion cannot be pumped or poured.
Considerable care and diligence is required in adding the base. A high, localized concentration will cause what is commonly referred to as "pH-shock". An overadjustment of the pH will disrupt the pH balance of the emulsion, resulting in an increased potential for later phase separation and curing problems. The same concerns follow use of a concentrated, rather than dilute, base solution.
Either under- or overdosage may lead to re-emulsification problems later. A base of insufficient strength will not completely de-encapsulate the gum, leaving the potential for further hydration after application. Overdosage with a stronger base such as a metal hydroxide, may lead to cationic interference with the clay surface. Later contact of the cured emulsion with water will initiate re-emulsification and destroy the film.
Several significant problems associated with thickeners of the prior art relate to the fact they are dry powders. Airborne particulates present the potential for respiratory problems among those so employed. The dust itself is highly explosive, given a source of ignition. As such, thickeners of the prior art have long been a concern to regulatory bodies such as the Federal Occupational Safety and Health Administration and the Environmental Protection Agency, as well as their state and local counterparts.
Production techniques and short-comings aside, phase separation is still a major problem of the prior art. Emulsion compositions prepared with such thickeners are unstable over time. Bitumen and clay settle during storage and transportation. Considerable mixing is required immediately prior to application, with the added expense of time and money.
Another major problem of the prior art is that emulsion compositions prepared with such thickeners tend to be non-thixotropic; that is to say, not shear-acceptable. Emulsions prepared in this way do not spread uniformly without interruption upon application of a shearing force and typically do not exhibit many other properties desired with surface coatings of this sort.
Another significant problem of the prior art is film performance. The non-uniform dispersion and non-homogeneity described above cause cured emulsion films of the prior art to have an uneven appearance, rather than the highly desired uniform dark black color. A related concern is decreased water resistance. The presence of unhydrated thickener will cause re-emulsification of the film upon contact with water.
Another significant problem is the required addition of biocides. Cellulose thickeners are, for the most part, vegetable matter. For example, cellulose gums are derived from various sources, including wood pulp and cotton. These materials support fungal and bacterial life, the bi-products of which are weakly acidic and may later "sour" the emulsion causing phase separation. As a preventative measure, synthetic biocides must be added during emulsion formulation and represent an extra production cost.
The overall cost of thickeners of the prior art is another major concern. Typically, bituminous emulsions produced for outdoor use are 0.5-2% by weight of a cellulose gum or latex, a relatively high concentration of a relatively expensive material. Furthermore, if higher concentrations of bitumen and clay are desired in the emulsion more thickener is needed to maintain the desired degree of suspension and viscosity. The real cost of the thickener is greatly increased when the pH-adjustment chemicals and biocides are considered.
In summary, a considerable number of drawbacks and problems exist in the art relating to clay-stabilized bituminous emulsions. There is a need for an improved emulsion composition.